Industrial boilers, such as oil-fired, coal-fired and trash-fired boilers in power plants used for electricity generation and waste incineration, as well as boilers used in paper manufacturing, oil refining, steel and aluminum smelting and other industrial enterprises, are huge structures that generate tons of ash while operating at very high combustion temperatures. These boilers are generally characterized by an enormous open furnace in a lower section of the boiler housed within walls constructed from heat exchanger tubes that carry pressurized water, which is heated by the furnace. An ash collection and disposal section is typically located below the furnace, which collects and removes the ash for disposal, typically using a hopper to collect the ash and a conveyor or rail car to transport it away for disposal.
A superheater section is typically located directly above the furnace, which includes a number of panels, also called platens or pendants, constructed from heat exchanger tubes that hang from the boiler roof, suspended above the combustion zone within the furnace. The superheater platens typically contain superheated steam that is heated by the furnace gas before the steam is transported to steam-driven equipment located outside the boiler, such as steam turbines or wood pulp cookers. The superheater is exposed to very high temperatures in the boiler, such as about 2800 degrees Fahrenheit [about 1500 degrees Celsius], because it is positioned directly above the combustion zone for the purpose of exchanging the heat generated by the furnace into the steam carried by the platens. The boiler also includes a number of other heat exchangers that are not located directly above the furnace, and for this reason operate at lower temperatures, such as about 1000-1500 degrees Fahrenheit [about 500-750 degrees Celsius]. These boiler sections may be referred to as a convection zone typically including one or more pre-heaters, re-heaters, superheaters, and economizers.
There is a high demand for thermal energy produced by these large industrial boilers, and they exhibit a high cost associated with shutting down and subsequently bringing the boilers back up to operating temperatures. For these reasons, the boilers preferably run continuously for long periods of time, such as months, between shut down periods. This means that large amounts of ash, which is continuously generated by the boiler, must be removed while the boiler remains in operation. Further, fly ash tends to adhere and solidify into slag that accumulates on high-temperature interior boiler structures, including the furnace walls, the superheater platens, and the other heat exchangers of the boiler. If the slag is not effectively removed while the boiler remains in operation, it can accumulate to such an extent that it significantly reduces the heat transfer capability of the boiler, which reduces the thermal output and economic value of the boiler. In addition, large unchecked accumulations of slag can cause huge chunks of slag to break loose, particularly from the platens, which fall through the boiler and can cause catastrophic damage and failure of the boiler.
The slag accumulation problem in many conventional boilers has been exacerbated in recent years by increasingly stringent air quality standards, which have mandated a change to coal with a lower sulphur content. This low-sulphur coal has a higher ash content and produces more tenacious slag deposits that accumulate more quickly and are more difficult to remove, particularly from the superheater platens. To combat this problem, the industry has developed increasingly sophisticated boiler cleaning equipment that operates continually while the boiler remains in operation. In particular, water cannons can be periodically used to clean the boiler walls in the open furnace section, and conventional steam sootblowers can be used to clean the heat exchangers. These steam sootblowers generally include lance tubes that are inserted into the boiler adjacent to the heat exchangers and operate like large pressure washers to clean the heat exchangers with steam blasts while the boiler remains in operation.
Conventional steam sootblowers have included rotating lance tubes that blast the steam in a corkscrew pattern to clean as wide an area as possible as the lance advances. In these superheaters, the platens are typically arranged in rows of panels, and therefore require a system of sootblowers that travel among and clean the various platens. However, slag deposits in some boiler superheaters have proven to be so tenacious that this type of steam cleaning is insufficient. For areas with slag deposits that resist steam cleaning, sootblowers that use water as the cleaning medium have been employed. A difficulty arises with the use of water as a cleaning fluid because the thermal shock imposed on the heat exchanger tubes is much greater when water is used as the cleaning fluid. Eventually, water shock can cause the heat exchanger tubes to crack and fail, which requires a major boiler renovation.
Water stress is such a serious issue that water cleaning should be kept to a minimum to avoid unnecessarily shortening the boiler's life. Furthermore, water cleaning tends to cause slag to be removed from the platens in fairly large sections, as the water penetrates the slang and flashes to steam, which blows chunks of slag away from the platen. Once a large chunk of slag has been removed, it is important that the now bare platen tubes not be shocked with subsequent water streams. It is also important that water cleaning, unlike steam, not be applied too close to the heat exchanger tubes to avoid cracking the tubes during the cleaning process.
The boiler cleaning problems described above have led to the proliferation of sootblowers, particularly in the superheater areas of boilers, because steam sootblowers are desirable for regularly-scheduled cleaning passes, whereas more closely controlled water sootblowers are desirable for occasional rigorous cleaning of areas encrusted with tenacious slag that resists steam cleaning. This dual-media cleaning need has led to the advent of dual-media sootblowers that have attempted to effectively deliver both steam and water as cleaning fluids. However, the objective of delivering both steam and water through a single lance has proved difficult to attain because water lances are typically tethered to water hoses, whereas steam lances rotate feely. In addition, water lances require greater precision and control than conventional steam lances afford, for example requiring independent control of the water streams and the ability to turn the water off at a particular water jet when that jet is positioned too close to a heat exchanger tube or directed at a structure that has already been successfully cleaned. Incorporating these capabilities into a water lance that also delivers steam as a cleaning fluid has not been successfully accomplished.
These difficulties are accentuated in the harsh environment of the interior of an operating industrial boiler. Sootblower lances can be quite long, such as 50 feet, depending on the particular boiler. Metal structures, such as tubes, hoses, couplers and nozzles experience extreme heat expansion and expansion-related stresses in this type of environment. Further, the need for long periods of active duty with very low failure rates is almost as critical for the boiler cleaning equipment as for the interior components of the boiler itself, which reduces the availability of complicated systems with intricate moving parts for interior boiler operations.
Accordingly, a continuing need exists for improved sootblowers and related automatic boiler cleaning systems for power plants. More specifically, a need exists for more effective cleaning systems for the platens in industrial boilers.